Macromolecules 1998, 31, 5943-5946
Maleic Anhydride Modified Polypropylene with Controllable Molecular Structure: New Synthetic Route via Borane-Terminated Polypropylene Bing Lu and T. C. Chung* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received March 24, 1998 Revised Manuscript Received May 27, 1998
Introduction. By far, maleic anhydride (MA) modified polyolefins are the most important class of functionalized polyolefins in commercial applications, due to the unique combination of low cost, high activity, and good processiblity. They are the general choice of material in improving compatibility, adhesion, and paintability of polyolefins. Among them, MA modified polypropylene (PP-MA)1 is the most investigated polymer, which has found applications in glass fiber reinforced PP,2 anticorrosive coatings for metal pipes and containers,3 metal-plastic laminates for structural use,4 multilayer sheets of paper for chemical and food packaging,5 and polymer blends6-8 (PP/polyamide and PP/ polyester). Usually, PP-MA was prepared by chemical modification of preformed PP under free radical conditions,9-12 as illustrated in eq 1.
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temperature, initiator, solvent, mixing, etc.) to improve the grafting efficiency and to minimize the undesirable side reactions. However, the inherent complexity of this reaction has greatly limited the progress of achieving PP-MA polymer with controllable molecular structure. The combination of mixed MA structure, reduced PP molecular weight and impurities has significantly reduced PP-MA ability as the interfacial agent in PP blends and composites. Overall, the PP-MA technology barely satisfies the current technological needs. Results and Discussion. In this paper, a new synthetic route is discussed, which is aimed to prepare MA-modified PP with controllable molecular structure, i.e., PP molecular weight and MA incorporated location and concentration. Equation 2 illustrates the reaction scheme.
(2)
(1)
As expected, this MA grafting reaction is accompanied by many potentially undesirable side reactions, such as β-scission, chain transfer, and coupling. The MA incorporation is usually inversely proportional to the resulting polymer molecular weight. In fact, it has been generally suggested that a significant portion of PP-MA polymers have a succinic anhydride group located at the polymer chain end,11,12 resulting from β-scission. Many experimental results,13 mainly found in patent literature, have been focused on the reaction conditions (i.e., * To whom all correspondence should be addressed.
The chemistry involves the usage of borane-terminated PP as the intermediate, which can be effectively prepared and transformed to MA-modified PP with controllable MA concentration and no detectable side reactions, i.e., chain degradation and cross-linking. Usually, the borane-terminated PP (PP-B) was prepared by hydroboration reaction of chain end unsaturated PP (u-PP), obtained from metallocene polymerization14 or thermal degradation15 of high molecular weight PP. In turn, PP-B polymer is selectively oxidized by oxygen16,17 to form a “stable” polymeric alkoxyl radical18 (associated with the boroxyl radical), which then initiates the free radical reaction of maleic anhydride. Due to the low tendency of homopolymerization of MA,19 a very low concentration of MA (possibly only a single unit) was incorporated in the PP structure to form MA-terminated PP (PP-b-MA). On the other hand, with the presence of styrene in the PP-B/MA mixture, the free radical graft from polymerization takes place to extend the PP chain end with an alternating styrene and MA (SMA) copolymer.20 In other words, a diblock copolymer PPb-SMA is obtained, containing both PP and SMA segments. The MA concentration in the copolymer is governed by the molecular weight of the SMA segment. Figure 1 compares IR spectra of the starting u-PP and two corresponding MA-modified PP polymers, i.e., PPb-MA and PP-b-SMA, which were prepared and fractionated by the procedures shown in the Experimental Section.
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Figure 1. Comparison of IR spectra of (a) PP-3, (b) PP-3-bMA, and (c) PP-3-b-SMA-3 polymers in the first reaction set in Table 1.
In Figure 1b, a new weak absorption band centered at 1710 cm-1, corresponding to the υCdO stretching mode of the acid form, indicates that a very low concentration of MA was incorporated into PP-b-MA and most of them were hydrolyzed during the sample handling. On the other hand, Figure 1c shows several new strong absorption peaks at 1860 and 1780 cm-1, corresponding to two υCdO vibrational stretching modes in succinic anhydride, at 900-950 cm-1, corresponding to the υC-H deformation in succinic anhydride, and at 1500, 700, 750, and 580 cm-1, corresponding to styrene υC-H deformation. Clearly, very high concentrations of MA groups were incorporated with styrene units in the PPb-SMA sample. The concentration of anhydride or/and acid was calculated by a standard industrial method (discussed in the Experimental Section), based on the IR carbonyl group absorption intensity and sample thickness. Table 1 summarizes the experimental conditions and results of two comparative reaction sets. In the first set, about 0.2 wt % MA was detected in the PP-3-b-MA sample, which is close to the theoretical value (0.16 wt %) of one MA unit per PP chain. With the addition of styrene, the incorporation of MA units dramatically increases, and the concentration is dependent on the reaction time (comparing samples PP3-b-SMA-2 and PP-3-b-SMA-3) and styrene concentration (comparing samples PP-3-b-SMA-1 and PP-3-bSMA-2). In the PP-3-b-SMA-3 sample, 25 wt % MA incorporation indicates about 50 wt % of SMA assuming the alternating copolymer segment existed in this diblock copolymer. In other words, the molecular weight of the SMA segment is about 5.3 × 104, similar to that of the PP segment. Similar results were concluded for the second set, despite the starting u-PP being of very high molecular weight (1.63 × 105) and thus having an extremely low concentration of reactive sites. Due to the sensitivity limit of FTIR spectroscopy, the MA concentration (0.06 wt %) in PP-8-b-MA is only an estimation, assuming one MA in each PP chain. However, with the styrene in the reaction mixture, the chain extension took place and the amount of incorporated MA units dramatically increased. IR spectra also show that the concentration of MA is also increased with the increases of styrene concentration and reaction time (comparing samples PP-8-b-SMA-1 and PP-8-b-SMA-2). Sample PP-8-b-SMA-2 contains a high concentration (20 wt %) of MA units with the estimated molecular weight
Macromolecules, Vol. 31, No. 17, 1998
>2.6 × 105, assuming an alternating SMA segment in the diblock copolymer. With the increase of SMA concentration in PP structure, the solubility of polymer gradually reduced. Only the samples containing MA units 3 wt % MA, such as PP-3-b-SMA-3, the reaction mixture was further subjected to Soxhlet extraction with refluxing xylene and acetone for 24 h to remove unreacted PP (0.182 g) and ungrafted SMA polymer (0.337 g), respectively. The xylene and acetone insoluble but thermally processible portion (2.014 g) is PP-b-SMA diblock copolymer. In general, the graft efficiency was very high, and the major portion (>80%) of PP was converted to PP-b-SMA diblock copolymer. The intrinsic viscosity of the polymer was measured in decahydronaphthalene (decalin) dilute solution at 135 °C with a Cannon-Ubbelohde viscometer. The viscosity molecular weight was calculated by the Mark-Houwink equation: [η] ) KMvR where K ) 1.05 × 10-5 and R ) 0.80.22 The MA concentration in the polymer was estmated by the IR spectrum with the following equation: MA wt % ) (k1A1780cm-1 + k2A1710cm-1)/d, wherein d is the film thickness, A1780cm-1 and A1710cm-1 are the peak absorbances, and k1 and k2 are the absorption constants for anhydride and acid, respectively. Both k1 and k2 were determined by calibration of the known
Figure 3. SEM micrographs of (a) the control blend with Nylon 11/PP ) 75/25 and (b) the reactive blend Nylon 11/PP3-b-MA/PP ) 75/5/20. (1500×). Micrographs reduced to 64% for publication.
commecial compounds (Uniroyal), assuming the absorption constants are independent of the incorporated MA structures. The melting point (Tm) and glass transition temperature (Tg) were measured by differential scanning calorimetetry (DSC) from the second run with a heating rate of 20 °C/min under nitrogen on a PerkinElmer DSC-7 instrument. Scanning electron microscopy (SEM) was used to examine the morphologies of polymer blends. Samples were prepared in a Brabender mixer by mixing various polymers at 240 °C for a few minutes. The mixture was then melt pressed to a film that was cryofractured in liquid N2 to obtain an undistorted view representative of the bulk material. The surface of the cross-section was sputtered with gold and then viewed with a Topcon International Scientific Intruments ISI-SX-40 using secondary electron imaging.
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Conclusion. It is both scientifically challenging and industrially important to develop a new method to prepare maleic anhydride modified polyolefins with controllable molecular structure. The borane-terminated polyolefin approach provides a promising route. In addition to the incorporation of the desirable amount of MA units, no detectable PP chain degradation and cross-linking reactions ensure the control of PP-b-MA molecular weight, similar to the starting PP. Despite the extremely low concentration of MA, PP-b-MA appears to be a very effective compatibilizer in the reactive PP/Nylon 11 blend. With the availability of various MAmodified PP polymers with relatively well-defined molecular structure, it is now possible to systematically study the correlation between the molecular structure, i.e., PP molecular weight and MA concentration, and their compatibility in PP blends and composites. Acknowledgment. The authors are thankful for the financial support from the Polymer Program of the National Science Foundation. References and Notes (1) Trivedi, B. C.; Culbertson, B. M. Maleic Anhydride; Plenum Press: New York, 1982. (2) Garagnai, E.; Marzola, R.; Moro, A. Mater. Plast. Elastomeri 1982, 5, 298. (3) Johnson, A. F.; Simms, G. D. Composites 1986, 17, 321. (4) Fukushima, N.; Kitagawa, Y.; Sonobe, T.; Toya, H.; Nagai, H. Eur. Patent Eur. Patent 78174.
Macromolecules, Vol. 31, No. 17, 1998 (5) Ashley, R. J. Adhesion 1988, 12, 239. (6) Felix, J. M.; Gatenholm, P. J. Appl. Polym. Sci. 1991, 42, 609. (7) Myers, G. E. J. Polym. Mater. 1991, 15, 21. (8) Majumdar, B.; Keskkula, H.; Paul, D. R. Polymer 1994, 35, 1386. (9) Lambla, M. In Comprehensive Polymer Science, First Supplement; Allen, G. Ed.; Pergamon Press: New York, 1982. (10) Priola, A.; Bongiovanni, R.; Gozzelino, G. Eur. Polym. J. 1994, 30, 1047. (11) Gaylord, N. G.; Mishra, M. K. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 23. (12) Heinen, W.; Rosenmoller, C. H.; Wenzel, C. B.; de Groot, H. J. M.; Lugtenburg, J.; van Duin, M. Macromolecules 1996, 29, 1151. (13) Ho, R. M.; Su, A. C.; Wu, C. H.; Chen, S. I. Polymer 1993, 34, 3264. (14) Tsutsui, T.; Mizuno, A.; Kashiwa, N. Polymer 1989, 30, 428. (15) Suwanda, D.; Lew, R.; Balke, S. T., J. Appl. Polym. Sci. 1988, 35, 1019. (16) Chung, T. C.; Janvikul, W.; Bernard, R.; Jiang, G. J. Macromolecules 1993, 27, 26. (17) Chung, T. C.; Janvikul, W.; Bernard, R.; Hu, R.; Li, C. L.; Liu, S. L.; Jiang, G. J. Polymer 1995, 36, 3565. (18) Chung, T. C.; Lu, H. L.; Janvikul, W. J. Am. Chem. Soc. 1996, 118, 705. (19) Russell, K. E.; Kelusky, E. C. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 2273. (20) Bruah, S. D.; Laskar, N. C. J. Appl. Polym. Sci. 1996, 60, 649. (21) Chung, T. C.; Lu, H. L.; Janvikul, W. Polymer 1997, 38, 1495. (22) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley-Interscience Publication: New York, 1989.
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